The present invention relates to a semiconductor light emitting device. Particularly, this invention relates to a semiconductor light emitting device with less crystal defects and higher performance.
FIG. 1 shows a conventional semiconductor light emitting device at its cross section. This semiconductor light emitting device consists of: a semiconductor substrate 11 of n-type gallium arsenide (GaAs); a transparent buffer layer 12 of n-type GaAs; a reflective layer 13 consisting of laminated two layers of indium aluminum phosphate (InAlP) GaAs (InAlP on GaAs); a lower clad layer 14 of n-type InGaAlP; an active layer 15 of undoped InGaAlP; an upper clad layer 16 of p-type InGaAlP; a transparent current diffusing layer 17 of p-type AlGaAs; a contact layer 18 of p-type GaAs; an upper electrode 19 and a lower electrode 20.
The buffer layer 12 prevents faults from being produced due to contamination of the surface of the semiconductor substrate 11 and also prevents the active layer 15 from being infected with the defects.
The reflective layer 13 reflects light emitted by the active layer 15 so that the emitted light does not enter the buffer layer 12 and the semiconductor substrate 11 made of light absorbent material. For this reason, the reflective layer 13 consists of semiconductor layers of InAlP and GaAs laminated with each other in a predetermined thickness. The layers of InAlP and GaAs have different refractive indices to the emitted light. The lower and upper clad layers 14 and 16 keep charge carriers injected into the active layer 15 to achieve high luminous efficiency.
The active-layer 15 consists of In1-y (Ga1-x Alx) Py. The components “x” and “y” and the layer construction determine energy gap. The active layer 15 emits light of wavelength corresponding to the energy gap when the injected carriers recombine with each other.
The current diffusing layer 17 diffuses current thereacross to take out the emitted light through whole region of the layer 17 not only directly below the upper electrode 19.
The current diffusing layer 17 is made of transparent material (p-type AlGaAs) that has a small absorbing coefficient to the emitted light wavelength.
The contact layer 18 makes better ohmic contact between the current diffusing layer 17 and the upper electrode 19.
The upper electrode 19 is a p-type electrode of Au layer which contains zinc. Through the upper electrode 19, a current is injected into a chip of the semiconductor light emitting device. The upper electrode 19 spreads the current over entire region of the semiconductor chip. Further, the upper electrode 19 is formed so as not to scatter the emitted light. The upper electrode 19 also acts as a bonding pad.
The lower electrode 20 is an n-type electrode of Au formed as a layer which contains germanium. The lower electrode 20 drains the current.
Another conventional semiconductor light emitting device is disclosed by Japanese Patent Laid-Open NO. 4 (1992)-212479. The conventional device is a light emitting diode with double hetero-configuration. In this device, an InGaAlP active layer is interposed between two clad layers.
Such a device with the InGaAlP active layer has required advanced epitaxy aiming at epitaxial growth with better crystallization, or fewer crystal defects. This epitaxial growth achieves higher device reliability. Further, such a light emitting device is fabricated with a molding material of low resin stress. The low-resin stress material reduces any decrease in luminescence after the light emitting device is driven.
However, it is very hard to keep crystal defects to a minimum in all layers grown by epitaxy. Device selection for quality in accordance with the number of crystal defects in all epitaxy-grown layers lowers device production yields. Further, low- and high-temperature degradation tests, after packaging the devices with molding resin, tend to produce much degradation in the resin packaged devices.